| Urine pregnancy test | Serum pregnancy test |
Minimum hCG level for a positive test | Qualitative test: 20 to 50 milli-int. units/mL, depending on test | Qualitative test: 5 to 10 milli-int. units/mL, depending on test Quantitative test: 1 to 2 milli-int. units/mL for an ultrasensitive test |
Causes of a false-negative test | - Performed too soon after conception; hCG concentration is below threshold for a positive test
- The hCG isoform measured is different from the hCG isoform in the sample (pertains mostly to urine tests)
- Hook effect due to extremely high hCG concentration (>500,000 milli-int. units/mL, these levels are most commonly seen in gestational trophoblastic neoplasia)
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Causes of a false-positive test | - Operator error in performing or interpreting the test, particularly with home pregnancy tests.
- Chemical pregnancy (ie, recent pregnancy loss very soon after implantation).
- Exogenous hCG administered as part of infertility treatment or for athletic performance. Exogenous hCG should be cleared by two weeks post-injection.
- HCG secretion from a tumor.
- Pituitary hCG secretion, typically in perimenopausal women.
- Interference with the assay by anti-animal antibodies, anti-hCG antibodies, or other substances (eg, high doses of biotin serum test positive but urine test is usually negative).
- Familial hCG syndrome (rare genetic condition)
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